1. Field of the Invention
The present invention relates to an anti-plating agent for use in the production of one-side plated steel sheet or strip by hot-dip process.
2. Description of the Prior Art
Recently, in the field of the steel sheet to be used for automobiles, domestic electric equipments, building materials and the like, it has been eagerly demanded to produce a so-called one-side plates steel sheet by plating only one side of a steel sheet to give the sheet a sufficiently high corrosion resistance and, at the same time, to improve the weldability of the sheet.
One-side plated sheet can be produced by the electroplating process, but the electroplating process is low in the plating speed and high in the production cost of the sheet. Therefore, the hot dip process is advantageously used in the production of the one-side plates steel sheet on a large scale.
There have hitherto been proposed various methods in the production of one-side plates steel sheet by the hot-dipping process. For example, the following methods are known in the production of zinc-plated steel sheet, that is, a method wherein two steel sheets are superposed and welded at the edge, and the welded steel sheets are plated, and then the welded edge is cut off; a method wherein molten zinc is plated on only one side of a steel sheet by the roll coating method, curtain-flow coating method or other particular method; a method wherein plating is effected on both sides of the steel sheet and then the plating layer on one side is removed by an electrolysis or grinding; a method wherein an anti-plating agent is applied beforehand on one side of the steel sheet and is removed after the plating; and a method wherein the plating is effected on only one side of the steel sheet by elevating or protruding the surface of the bath by means of ultrasonic wave or an electromagnetic pump.
These known methods, however, are generally impractical to carry out and raise the cost of production uneconomically. For these reasons, only few of them are put into production in the commercial scale.
As the conventional one-side zinc plating methods employing anti-plating agent, the following technics have been known. Namely, Japanese patent application publication No. 7,112/64 discloses the use of water-glass, while Japanese patent application publication No. 4,204/64 teaches the use of an aqueous slurry of a mixture of CaO, MgO and alkali metaborate. Japanese patent laid-open No. 48.029/78 teaches the use of an aqueous slurry consisting mainly of alkali metal silicate and ammonium silicate, while Japanese patent application publication No. 8,101/76 discloses a plating prohibitor consisting essentially of a silicon resin. The use of an aqueous slurry of scale-like synthetic silicon compound is disclosed in Japanese patent laid-open No. 64,026/79. Also, U.S. Pat. No. 3,121,019 discloses the use of alkali earth metal oxides.
These methods, however, still have the following drawbacks. That is, in the continuous hot-dip metal plating process, wherein a steel sheet with an anti-plating film coated thereon is annealed in reducing atmosphere (usually at 700.degree. C.) just before the plating by the hot-dipping, the anti-plating film coating the sheet surface decomposes or partly exfoliates from the steel sheet surface during the annealing. It is, therefore, difficult to completely prevent one side of the sheet from being plated. Moreover, the steel is sometimes oxidized in the air after plating, and a troublesome step is required in order to reduce or mechanically remove the oxide. Further, the film formed on one side of the steel sheet by the coating agent is generally poor in the peeling property, and it is almost impossible to remove the film completely without deteriorating the appearance of the steel sheet surface. In addition, the removal of the film is usually expensive.
As the anti-plating agents which can effectively prevent the plating and attaching of molten metal onto the coating layer while exhibiting good peeling property, the present inventors have already proposed an aqueous slurry containing four constituents: namely, magnesia, boric acid, an alkali and alkali silicate, in Japanese patent laid-open No. 146,730/77. Subsequently, the inventors proposed in Japanese Patent Laid-open No. 119,157/80 and aqueous slurry of 5-component system containing, in addition to the four constituents mentioned above, titania or titanium hydroxide.
These anti-plating agents, however, still suffered the following drawbacks. Namely, in the continuous hot-dip zinc plating method having the step of annealing in a reducing atmosphere in advance to the plating, the zinc attaches to the coating layer of the anti-plating agent when the speed of pulling out of the steel from the molten zinc bath is too high or when the annealing is conducted at a temperature above the A.sub.1 transformation temperature aiming at achieving a higher workability of the zinc-plated steel sheet. The attaching of the zinc to the coating layer causes not only the wasteful use of the zinc but also a contamination of the production line due to peeling off of the zinc when the steel sheets move along the path of the production line.